Aerial refueling is a common method for extending the flight time and range of an aircraft and typically involves a tanker aircraft, which is loaded with excess fuel, transferring fuel to one or more trailing or receiver aircraft. Several types of in-flight refueling systems have emerged over the years.
One type of refueling system is the boom type disclosed in U.S. Pat. No. 2,663,523. A rigid refueling boom mounted below the tail extends outward from the belly of the tanker and is controlled by an operator located within the tanker. The receiver aircraft positions behind the tanker, permitting the operator on the tanker to direct the end of the boom into the refueling port on the receiver aircraft and to initiate refueling.
An alternate type of refueling system is disclosed in U.S. Pat. No. 3,008,674. The system incorporates a detachable refueling pod which is mounted below the main wing of the aircraft. A hose extends from the aft portion of the pod and has a drogue attached to its trailing end which stabilizes the hose in the air stream and thereby facilitates interconnection by the receiving aircraft. By mounting a pod under each wing, two receiving aircraft can refuel at the same time from opposite sides of the tanker. Additionally, mounting the pod under the wing permits the refueling hose and drogue to trail behind the aircraft, lowering the likelihood that the hose or drogue will impact the fuselage or tail.
One drawback to a wing-mounted refueling pod is that the weight of the pod produces high bending loads in the Wing. Consequently, substantial structural modifications must be made to the wing to accommodate the increased loads. The increased bending of the wings caused by the pods tends to reduce the fatigue life of the wing. Furthermore, the location of the pods on the wings produces an excessive amount of drag, which reduces the fuel efficiency of the tanker aircraft.
Another deficiency with the wing-mounted refueling pods is the difficulty that the receiver aircraft sometimes encounters when attempting to engage with the drogue. Referring to FIG. 1a, a rear view of a tanker aircraft 10 is shown which includes a main wing 12, a stabilizer 14, a fuselage 16 and engines 18. Also shown is a wing-mounted refueling pod 20 with a refueling hose 22 extended therefrom and a drogue 24 attached to the trailing end of the hose. A receiver aircraft 26 is depicted approaching the tanker aircraft for in-flight refueling. In order to transfer fuel from the tanker aircraft to the receiver aircraft, the drogue must connect with a probe or refueling port on the receiver aircraft. In a wing-mounted refueling system, the tanker aircraft has no control over the motion of the drogue and refueling hose and, therefore, the drogue is free to travel +/-30 degrees above, below and to the sides of the wing. Hence, in order for the receiver aircraft to engage with the drogue, it must be maneuvered in such a manner so as to track or chase the motions of the drogue.
The situation is further complicated by the airflow over the wing. As shown in FIG. 1b, the airflow over the wing tends to drive the refueling hose 22 and drogue 24 towards the fuselage 16. Accordingly, the receiver aircraft 26 must maneuver in the direction of the drogue 24. As the receiver aircraft 26 approaches the fuselage 16 the exhaust from the engines 18 (indicated in FIG. 1c by numeral 28) causes the receiver aircraft 26 to veer to the left, driving it outboard. As the receiver aircraft 26 approaches the outer tip of the wing 12, the wing tip vortices, generated by the turbulent air flow around the wing tip, depicted by numeral 29 in FIG. 1d, force the receiver aircraft 26 to veer back to the right. This procedure continues until the receiver aircraft. 26 finally engages with the refueling hose 22 and drogue 24. It is common to make several attempts before the receiver aircraft 26 engages with the refueling hose 22 and drogue 24. Accordingly, in-flight refueling from a wing-mounted system can be an extremely difficult and dangerous undertaking.
In an attempt to reduce the difficulty associated with in-flight refueling, drogue configurations, such as the type shown in U.S. Pat. No. 2,596,455, have been created which attempt to maintain the refueling hose in a relatively stable flight path. These configurations, however, have not proven satisfactory.
A need therefore exists for an improved refueling system which permits a receiver aircraft to easily and safely engage with a drogue and refueling hose, and which system minimizes the drag and bending loads imposed on the tanker aircraft.